On-board fuel cell system and method of controlling the same

ABSTRACT

An on-board fuel cell system adapted to be installed on a motor vehicle includes a main passage connecting a hydrogen-gas storage device with an inlet of a fuel cell, a circulation passage that connects an outlet of the fuel cell with a first point in the main passage, a pump disposed in the circulation passage, and a bypass passage that connects a second point between the outlet of the storage device and the first point, with a third point located in the circulation passage between the outlet of the fuel cell and the pump. During a normal operation condition of the system, the hydrogen gas flows from the storage device to the fuel cell through the main passage, and hydrogen gas discharged from the fuel cell returns to the main passage through the circulation passage. When the pressure of the hydrogen gas is lower than a reference pressure, the pump operates to draw the hydrogen gas out of the storage device and feed the hydrogen gas from the main passage to the circulation passage through the bypass passage, and to the fuel cell through the main passage.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2001-010519 filed onJan. 18, 2002, including the specification, drawings and abstract, isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention generally relates to an on-board fuel cell system suitedfor installation on a motor vehicle, such as an automobile, and alsorelates to a method of controlling such an on-board fuel cell system.

2. Description of Related Art

Fuel cells are known as generating electric power by using hydrogen gassupplied from a high-pressure hydrogen-gas tank or a hydrogen-gasabsorbing alloy tank. The fuel cells, which exhibit high energyefficiency, are highly expected to be used as a power source forelectric vehicles, and the like.

When such a fuel cell is used as a power source for a vehicle, there isa need to install, on the vehicle, a fuel cell system that includes notonly the fuel cell, but also a hydrogen gas supply, such as ahigh-pressure hydrogen tank or a hydrogen-gas absorbing alloy tank, anda hydrogen-gas passage through which the hydrogen gas is fed from thehydrogen gas supply to the fuel cell.

For installation on the vehicle, therefore, the fuel cell system ispreferably made as compact as possible in size and as light as possiblein weight. Furthermore, the fuel cell system, which handles a highlycombustible or flammable hydrogen gas, is required to assure a highdegree of safety.

SUMMARY OF THE INVENTION

It is therefore one object of the invention to provide an on-board fuelcell system which is lightweight and is compact in size for installationon a vehicle, and which assures a high degree of safety.

To accomplish the above and/or other object(s), there is providedaccording to a first aspect of the invention an on-board fuel cellsystem adapted to be installed on a motor vehicle, which includes (a) ahydrogen-gas storage device including a hydrogen-gas absorbing alloycapable of absorbing or releasing a hydrogen gas, (b) a fuel cell thatis supplied with the hydrogen gas discharged from the hydrogen-gasstorage device, so as to generate electric power, while discharging aremaining portion of the hydrogen gas, (c) a first passage that connectsan outlet of the hydrogen-gas storage device with an inlet of the fuelcell, and allows the hydrogen gas discharged from the hydrogen-gasstorage device to flow therethrough to be supplied to the fuel cell, (d)a second passage that connects an outlet of the fuel cell with a firstpoint in the first passage, and allows the hydrogen gas discharged fromthe fuel cell to flow therethrough to be returned to the first passage,(e) a pump that is disposed in the second passage, and is operable toforce the hydrogen gas in the second passage to flow toward the firstpoint in the first passage, (f) a third passage that connects a secondpoint located in the first passage between an outlet of the hydrogen-gasstorage device and the first point, with a third point located in thesecond passage between the outlet of the fuel cell and the pump, thethird passage allowing the hydrogen gas diverting from the first passageto flow therethrough to be fed to the second passage, (g) a first valvethat is disposed in the first passage between the second point and thefirst point, and is able to permit and inhibit flow of gas therethroughupon opening and closing thereof, respectively, (h) a second valve thatis disposed in the second passage between the outlet of the fuel celland the third point, and is able to permit and inhibit flow of gastherethrough upon opening and closing thereof, respectively, (i) a thirdvalve that is disposed in the third passage, and is able to permit andinhibit flow of gas therethrough upon opening and closing thereof,respectively, and (j) a controller that controls the pump and the first,second and third valves. When a pressure of the hydrogen gas dischargedfrom the hydrogen-gas storage device is higher than a referencepressure, the controller opens the first and second valves and closesthe third valve, so that the hydrogen gas discharged from thehydrogen-gas storage device is supplied to the fuel cell through thefirst passage, and the hydrogen gas discharged from the fuel cell isreturned to the first passage through the second passage, with the pumpcirculating the hydrogen gas. When the pressure of the hydrogen gasdischarged from the hydrogen-gas storage device is lower than thereference pressure, on the other hand, the controller closes the firstand second valves and opens the third valve, and causes the pump to drawthe hydrogen gas out of the hydrogen-gas storage device and feed thehydrogen gas from the first passage to the second passage through thethird passage, so that the hydrogen gas is supplied from the secondpassage to the fuel cell through the first passage.

According to a second aspect of the invention, there is provided amethod of controlling the on-board fuel cell system constructed asdescribed just above, which method includes the steps of (a) determiningwhether a pressure of the hydrogen gas discharged from the hydrogen-gasstorage device is equal to or higher than a reference pressure, (b) whenthe pressure of the hydrogen gas is higher than the reference pressure,opening the first and second valves and closing the third valve, so thatthe hydrogen gas discharged from the hydrogen-gas storage device issupplied to the fuel cell through the first passage, and the hydrogengas discharged from the fuel cell is returned to the first passagethrough the second passage, with the pump circulating the hydrogen gas,and (c) when the pressure of the hydrogen gas is lower than thereference pressure, closing the first and second valves while openingthe third valve, and causing the pump to draw the hydrogen gas out ofthe hydrogen-gas storage device and feed the hydrogen gas from the firstpassage to the second passage through the third passage, so that thehydrogen gas is supplied from the second passage to the fuel cellthrough the first passage.

In the on-board fuel cell system or control method thereof as describedabove, a single pump is used for circulating the hydrogen gas during anormal operating condition of the fuel cell system in which the pressureof the hydrogen gas is higher than the reference pressure level, and isalso used for drawing the hydrogen gas when the pressure of the hydrogengas is lower than the reference level, for example, upon a start of thefuel cell system at a low temperature. Thus, the same pump is used forthe two purposes, i.e., for circulating the hydrogen gas and drawing thegas from the storage device, resulting in reduced space required forinstalling the fuel cell system on the vehicle, and a reduction in theweight of the system, as compared with the case where separate pumps orother devices are provided for the above two purposes.

By circulating the hydrogen gas with the pump during a normal operatingcondition of the fuel cell system, an apparent flow rate (i.e., amountand flow speed) of the hydrogen gas supplied to the fuel cell isincreased, which is advantageous in terms of supply of hydrogen to thefuel cell, and leads to an increased output voltage of the fuel cell.Furthermore, even if impurities, such as nitrogen, leak into thehydrogen gas in the fuel cell, the impurities are uniformly distributedover the entire length of a hydrogen-gas flow system including the firstand second passages, with the hydrogen gas circulating in the flowsystem. Thus, the impurities are prevented from remaining in the fuelcell and causing a problem with the power generating operation of thefuel cell.

Although the hydrogen gas is less likely to be released from thehydrogen-gas absorbing alloy in the hydrogen-gas storage device upon alow-temperature start of the fuel cell system, the pump is able to drawthe hydrogen gas out of the storage device, so that the fuel cell canstart operating in a steady state within a relatively short time.

For example, the first and second valves as indicated above may beintegrated into a flow-path switching valve.

According to a third aspect of the invention, there is provided anon-board fuel cell system adapted to be installed on a motor vehicle,which system includes: (a) a hydrogen-gas supply device that supplies ahydrogen gas; (b) a fuel cell that is supplied with the hydrogen gasdischarged from the hydrogen-gas supply device, so as to generateelectric power, while discharging a remaining portion of the hydrogengas, the fuel cell having a plurality of channels through which thehydrogen gas flows; (c) a first passage that connects an outlet of thehydrogen-gas supply device with an inlet of the fuel cell, and allowsthe hydrogen gas discharged from the hydrogen-gas supply device to flowtherethrough to be supplied to the fuel cell; (d) a second passage thatconnects an outlet of the fuel cell with a particular point in the firstpassage, and allows the hydrogen gas discharged from the fuel cell toflow therethrough to be returned to the first passage; (e) a pump thatis disposed in the second passage, and is operable to force the hydrogengas in the second passage to flow toward the particular point in thefirst passage; and (f) a controller that controls the hydrogen-gassupply device and the pump. Upon a start of the fuel cell system, thecontroller causes the hydrogen-gas supply device to deliver the hydrogengas, while driving the pump so as to induce flow of the hydrogen gasthrough at least a portion of the first and second passages and thechannels of the fuel cell, thereby to mix impurities existing in thechannels with the hydrogen gas delivered from the hydrogen-gas supplydevice to provide a homogeneous mixture thereof.

According to a fourth aspect of the invention, there is provided amethod of controlling the on-board fuel cell system constructed asdescribed just above, which method includes the steps of: (a) causingthe hydrogen-gas supply device to deliver the hydrogen gas upon a startof the fuel cell system; and (b) driving the pump so as to induce flowof the hydrogen gas through at least a portion of the first and secondpassages and the channels of the fuel cell, thereby to mix impuritiesexisting in the channels with the hydrogen gas delivered from thehydrogen-gas supply device to provide a homogeneous mixture thereof.

Upon a start of the fuel cell system, impurities, such as nitrogen, maybe contained in hydrogen-gas channels within the fuel cell. By merelypermitting hydrogen gas to flow through the hydrogen-gas channels, ittakes a long time by the time at which the output voltage of the fuelcell reaches a desired voltage level. In the on-board fuel cell systemor the control method as described above, therefore, the hydrogen gas isdelivered from the hydrogen-gas supply device upon a start of the fuelcell system, and at the same time the pump is driven so as to induce orcreate strengthened flow of the hydrogen gas through the appropriatehydrogen-gas channels, thereby to mix impurities existing in thechannels with the hydrogen gas delivered from the hydrogen-gas supplydevice to provide a homogeneous mixture thereof.

With the remaining impurities, such as nitrogen, being uniformly mixedwith the hydrogen gas delivered from the hydrogen-gas supply device asdescribed above, the output voltage of the fuel cell is immediatelyraised to the desired level, and desired power can be supplied to theload connected to the fuel cell.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and/or further objects, features and advantages of theinvention will become more apparent from the following description ofpreferred embodiments with reference to the accompanying drawings, inwhich like numerals are used to represent like elements and wherein:

FIG. 1 is a schematic diagram showing the construction of an on-boardfuel cell system according to a first preferred embodiment of theinvention;

FIG. 2 is a cross-sectional view of one example of a gas-liquidseparator for use in the fuel cell system of FIG. 1;

FIG. 3 is a flowchart illustrating an example of a control routineexecuted by a controller when the fuel cell system of FIG. 1 is started;

FIG. 4 is a schematic diagram showing the construction of an on-boardfuel cell system according to a second preferred embodiment of theinvention; and

FIG. 5 is a flowchart illustrating an example of a control routineexecuted by a controller when the fuel cell system of FIG. 4 is started.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Some exemplary preferred embodiments of the invention will be describedin detail with reference to the accompanying drawings.

First Embodiment

FIG. 1 schematically shows the construction of an on-board fuel cellsystem according to a first embodiment of the invention. The fuel cellsystem of this embodiment is installed on a motor vehicle, such as anautomobile. The fuel cell system primarily includes a fuel cell 100 thatgenerates electric power by using hydrogen gas supplied thereto, and ahydrogen-gas absorbing alloy tank 200 from which hydrogen gas issupplied to the fuel cell 100.

The fuel cell 100 is supplied with oxidizing gas (e.g., air) includingoxygen as well as the hydrogen gas including hydrogen. The hydrogen gasand the oxidizing gas thus supplied undergo electrochemical reactions asrepresented by formulas (1) and (2) below at a hydrogen electrode and anoxygen electrode, respectively, so that the fuel cell 100 generateselectric power.

More specifically, a reaction as expressed by the following formula (1)occurs on the side of the hydrogen electrode when the hydrogen gas issupplied to the hydrogen electrode, and a reaction as expressed by thefollowing formula (2) occurs on the side of the oxygen electrode whenthe oxidizing gas is supplied to the oxygen electrode. Thus, the fuelcell 100 as a whole performs an electrochemical reaction as expressed bythe following formula (3).H₂→2H⁺+2e⁻  (1)2H⁺+2e⁻+(½)O₂→H₂O   (2)H₂+(½)O₂→H₂O   (3)

In the vehicle incorporating the fuel cell system 100 as a power source,an electric motor (not shown) is driven by the electric power generatedby the fuel cell system 100. The resulting torque of the electric motoris then transmitted to an axle (not shown), thus producing driving forceof the vehicle.

The fuel cell 100 has a stacked structure formed by stacking orlaminating a plurality of unit cells together. Each of the unit cellsincludes an electrolyte layer (not shown), a pair of diffusionelectrodes (not shown) in the form of the hydrogen electrode and theoxygen electrode, and two separators. The hydrogen and oxygen electrodesare superposed on opposite major surfaces of the electrolyte layer, andthe separators are superposed on the outer surfaces of the hydrogen andoxygen electrodes. Grooves or recesses are formed in the oppositesurfaces of each of the separators, such that unit-cell gas channels areformed between the separators and the hydrogen electrode and oxygenelectrode interposed therebetween. In operation, the hydrogen gassupplied to the fuel cell 100 flows through the unit-cell gas channelsformed between the separators and the hydrogen electrodes, and theoxidizing gas supplied to the fuel cell 100 flows through the unit-cellgas channels formed between the separators and the oxygen electrodes.

In the meantime, the hydrogen-gas absorbing alloy tank 200 stores ahydrogen-gas absorbing alloy (not shown) therein. In general, thehydrogen-gas absorbing alloy undergoes an endothermic reaction whenheated, thereby to release hydrogen, and undergoes an exothermicreaction when cooled, thereby to absorb hydrogen. In order to takehydrogen out of the hydrogen-gas absorbing alloy, therefore, thehydrogen-gas absorbing alloy contained in the hydrogen-gas absorbingalloy tank 200 is heated by a suitable heat exchanger (not shown).

The hydrogen-gas absorbing alloy may deteriorate if it reacts withimpurities. Therefore, high-purity hydrogen is stored in thehydrogen-gas absorbing alloy tank 200.

The fuel cell system according to the first embodiment of the inventionfurther includes a hydrogen-gas flow system for permitting flow of thehydrogen gas through the system, an oxidizing-gas flow system forpermitting flow of the oxidizing gas through the system, and acontroller 50.

The hydrogen-gas flow system includes a main-stream passage 401 thatextends from an outlet of the hydrogen-gas absorbing alloy tank 200 toan inlet of the fuel cell 100, a circulation passage 403 through whichhydrogen gas flows from an outlet of the fuel cell 100 back to themain-stream passage 401 through a pump 410 which will be describedlater, and a bypass passage 405 that diverges from the main-streampassage 401 and is connected to the circulation passage 403. Thehydrogen-gas flow system further includes a drain passage 407 fordischarging impurities contained in the hydrogen gas circulating throughthe hydrogen-gas flow system, and a relief passage 409 for discharging acertain amount of hydrogen gas when the pressure of the hydrogen gas inthe flow system becomes excessively high.

In the main-stream passage 401, a shut-off valve 202 is disposed at theoutlet of the hydrogen-gas absorbing alloy tank 200, and a pressuresensor 400, a shut-off valve 402, and a pressure reducing valve 404 aredisposed at selected positions in the passage 401. In the circulationpassage 403, a shut-off valve 104 is disposed at the inlet of the fuelcell 100, and a gas-liquid separator 406, a shut-off valve 408, and thepump 410 are disposed at selected positions in the passage 403.Furthermore, a shut-off valve 412 is disposed in the bypass passage 405,and a shut-off valve 414 is disposed in the drain passage 407, while arelief valve 416 is disposed in the relief passage 409.

On the other hand, the oxidizing-gas flow system includes anoxidizing-gas supply passage 501 through which oxidizing gas is suppliedto the fuel cell 100 and an oxygen-off-gas drain passage 503 throughwhich oxygen off-gas is discharged from the fuel cell 100.

In the oxidizing-gas flow system, an air cleaner 502, a compressor 504,and a humidifier 506 are disposed at selected positions in theoxidizing-gas supply passage 501. Also, a gas-liquid separator 508 and acombustor 510 are disposed at selected positions in the oxygen-off-gasdrain passage 503.

The controller 50 receives a signal from the pressure sensor 400 fordetecting the pressure in the main-stream passage 401, and controlsoperation of each of the valves 102, 104, 202, 402, 408, 412, 414, thepump 410 and the compressor 504. In FIG. 1, control lines that indicateconnection between the controller 50 and each of the above componentsare not shown for the sake of simplicity.

First, flow of the oxidizing gas in the oxidizing-gas flow system willbe briefly described. The compressor 504 is driven by the controller 50so as to introduce a suitable amount of air in the atmosphere into theoxidizing-gas supply passage 501, for use as oxidizing gas in the fuelcell 100. The air thus introduced is purified by the air cleaner 502, ispassed through the humidifier 506, and is then supplied to the fuel cell100 through the oxidizing-gas supply passage 501. The oxidizing gassupplied to the fuel cell 100 is used for the above-describedelectrochemical reaction, and is then discharged form the fuel cell 100as the oxygen off-gas. The oxygen off-gas discharged from the fuel cell100 flows through the oxygen-off-gas drain passage 503, while passingthrough the gas-liquid separator 508 (which will be described in detaillater) and the combustor 510. Then, the oxygen off-gas is finallydischarged or released to the atmosphere.

Next, flow of the hydrogen gas in the hydrogen-gas flow system will bedescribed in detail. The controller 50 controls operation of theshut-off valve 202 disposed in the hydrogen-gas absorbing alloy tank200, and the shut-off valves 102, 104 disposed in the fuel cell 100,such that the valves 202, 102, 104 are normally open during operationsof the fuel cell system, and are closed when the fuel cell system stopsbeing operated.

When the fuel cell system is in a normal operating condition, thecontroller 50 controls the shut-off valves such that the shut-off valve402 of the main-stream passage 401 and the shut-off valve 408 of thecirculation passage 403 as well as the shut-off valves 202, 102, 104 asdescribed above are opened, while the shut-off valve 412 of the bypasspassage 405 and the shut-off valve 414 of the drain passage 407 areclosed. The relieve valve 416 is normally closed under the control ofthe controller 50 except when a pressure of the hydrogen gas in thepassage 401 become excessively high, as will be described later. Thepressure sensor 400 detects the pressure of the hydrogen gas dischargedfrom the hydrogen-gas absorbing alloy tank 200.

In the normal operating condition of the fuel cell system, thehydrogen-gas absorbing alloy contained in the hydrogen-gas absorbingalloy tank 200 is heated by a suitable heat exchange system so as todischarge hydrogen gas. The discharged hydrogen gas is then supplied tothe fuel cell 100 through the main-stream passage 401. The hydrogen gasfed to the fuel cell 100 is consumed through the electrochemicalreaction in the fuel cell 100, and is then discharged from the fuel cell100 as a hydrogen off-gas. The discharged hydrogen off-gas is returnedto the main-stream passage 401 through the circulation passage 403, sothat the off-gas is supplied to the fuel cell 100 again to be reused. Atthis time, the pump 410 disposed in the circulation passage 403 isdriven to force or urge the hydrogen off-gas passing through thecirculation passage 403 to be fed to the main-stream passage 401. Inthis manner, the hydrogen gas circulates through the main-stream passage401 and the circulation passage 403 when the fuel cell system is in thenormal operating condition.

Since the hydrogen off-gas is returned to the main-stream passage 401for circulation, apparent amount and flow speed of the hydrogen gassupplied to the fuel cell 100 are increased, even though the same amountof hydrogen is used or consumed in the fuel cell 100. This arrangementis advantageous in terms of supply of hydrogen to the fuel cell 100, andleads to an increase of an output voltage of the fuel cell 100.

In the fuel cell 100, impurities, such as nitrogen, contained in theoxidizing gas may leak from the oxygen electrode side to the hydrogenelectrode side through the electrolyte layer. Accordingly, if thehydrogen gas is not circulated in the fuel cell system, the impuritiesmay be accumulated on the hydrogen electrodes in a downstream portion ofthe fuel cell 100. The range of accumulation of the impurities on thehydrogen electrodes expands with time, which may cause a problem withthe power generating operation of the fuel cell 100, resulting in areduction in the output voltage of the fuel cell 100. In this embodimentin which the hydrogen gas is circulated as described above, on the otherhand, the impurities may be uniformly distributed over the entire lengthof the hydrogen-gas passage, and the fuel cell system is free from theabove-described problem caused by accumulation of the impurities.

The controller 50 controls operation of the pump 410 so that the flowrate or speed of the hydrogen gas through the circulation passage 403varies depending upon an amount of consumption of the electric powergenerated by the fuel cell 100.

The hydrogen gas discharged from the hydrogen-gas absorbing alloy tank200 has a considerably high pressure that is not larger than 1 MPa. Ifthe discharged hydrogen gas is directly fed to the fuel cell 100, thefuel cell 100 may deteriorate due to the high pressure of the hydrogengas. In view of this problem, the pressure reducing valve 404 disposedin the main-stream passage 401 is actuated to reduce the pressure of thehydrogen gas from 1 MPa to an appropriate level to be fed to the fuelcell 100, i.e., to a level ranging from about 0.2 MPa to about 0.3 MPa.Thus, the hydrogen gas whose pressure has been appropriately reduced isfed to the fuel cell 100.

In the fuel cell 100, water (H₂O) is produced on the oxygen electrodeside according to the reaction as expressed by the above-indicatedformula (2). The water in the form of vapor passes through theelectrolyte layer, from the oxygen electrode side to the hydrogenelectrode side. For this reason, the hydrogen off-gas discharged fromthe fuel cell 100 is wet and contains a significantly large amount ofmoisture. If the hydrogen off-gas is directly returned to themain-stream passage 401 via the pump 410, the moisture or watercontained in the hydrogen off-gas is not vaporized sufficiently. As aresult, the mixture of the hydrogen gas and the moisture, i.e., agas-liquid mixture is fed to the fuel cell 100. The moisture supplied tothe fuel cell 100 may adhere to walls in the unit cells of the fuel cellstack, possibly resulting in clogging of hydrogen-gas channels in thefuel cell 100. If the hydrogen-gas channels are clogged or closed due tothe moisture contained in the hydrogen gas, and flow of the hydrogen gasthrough the channels is restricted or stopped, the output voltage of theunit cells of the fuel cell 100 is reduced, resulting in a reduction inelectric power generated by the fuel cell 100 as a whole.

In view of the above-described problem, the gas-liquid separator 406disposed in the circulation passage 403 functions to separate themoisture contained the hydrogen off-gas into a liquid component and agas component (i.e., water vapor). The separator 406 then removes theliquid component of the hydrogen off-gas, and feeds only the gascomponent (i.e., water vapor) to the pump 410 along with other gases.With the gas-liquid separator 406 thus provided, only the gas componentof the moisture contained in the hydrogen gas, rather than thegas-liquid mixture, is supplied to the fuel cell 100. Thus, theprovision of the separator 406 eliminates the possibility ofdeterioration of the power generating operation of the fuel cell 100 dueto the moisture supplied to the fuel cell 100.

FIG. 2 shows, in cross section, one example of a gas-liquid separatorthat may be used in the fuel cell system of FIG. 1. A hydrogen off-gascontaining a large amount of moisture is introduced into a cylinder 604through an inlet 602 of the gas-liquid separator. The hydrogen off-gasintroduced from the inlet 602 falls or descends along an inner wall ofthe cylinder 604 while rotating spirally. In this process, the moisturecontained in the hydrogen off-gas is concentrated. Namely, the liquidcomponent of the moisture in the form of droplets adheres to the innerwall of the cylinder 604, and the droplets fall along the inner wall ofthe cylinder 604, to be collected in a liquid storage 608. On the otherhand, the gas component of the moisture (i.e., water vapor) isdischarged from an outlet 606 through a gas passage 610 together withother gas components in the hydrogen off-gas. In this manner, themoisture contained in the hydrogen off-gas can be separated into theliquid component and the gas component as described above.

The amount or level of water collected in the liquid storage 608 may bedetected by a level sensor (not shown) or the like. When the levelsensor detects a predetermined amount of water collected in the liquidstorage 608, a drain mechanism (not shown) is actuated to automaticallyopen a cock 612, to thereby discharge the collected water through thecock 612.

As discussed above, the hydrogen gas is circulated in the hydrogen-gasflow system so that the impurities contained in the hydrogen gas areuniformly distributed over the entire length of the circulation path.Even with the hydrogen gas thus homogenized, the fuel cell 100experiences constant leakage of the impurities from the oxygen electrodeside to the hydrogen electrode side through the electrolyte layer, andtherefore the concentration of the impurities in the hydrogen gasgradually increases over a long period of use. As a result, theconcentration of hydrogen in the hydrogen gas gradually decreases withtime, which may give rise to an adverse effect on the power generatingoperation of the fuel cell 100.

In view of the above problem, the shut-off valve 414 is provided in thedrain passage 407 that diverges from the circulation passage 403. Theshut-off valve 414 is periodically opened to discharge the circulatinghydrogen gas containing the impurities. The discharged hydrogen gascontaining the impurities is replaced with pure hydrogen gas newlysupplied from the hydrogen-gas absorbing alloy tank 200. Thisarrangement makes it possible to lower the concentration of theimpurities in the circulating hydrogen gas, while increasing theconcentration of hydrogen in the hydrogen gas, whereby the fuel cell 100become able to appropriately perform its power generating operation.

As described above, the fuel cell 100 also experiences leakage of watervapor from the oxygen electrode side to the hydrogen electrode sidethrough the electrolyte layer. The water vapor may be concentrated andadheres to the walls of the unit cells, depending upon an operationtemperature of the fuel cell 100, and the resulting moisture or watermay restrict or stop flow of the hydrogen gas through the unit cells. Ifthe shut-off valve 414 is opened to discharge the hydrogen gas in thissituation, rapid flow of the hydrogen gas takes place due to adifference between the pressure in the hydrogen-gas flow system and theatmospheric pressure, and the moisture adhering to the unit cells can beblown off, utilizing the stream of the hydrogen gas.

The opening of the shut-off valve 414 during the power generatingoperation of the fuel cell 100 causes a temporary or instantaneous dropof the output voltage of the fuel cell 100, but does not result in asignificant reduction in the output voltage. The shut-off valve 414 ispreferably kept opened for 1 sec. or shorter, more preferably for about500 msec.

The hydrogen gas discharged from the shut-off valve 414 is fed to theoxygen off-gas drain passage 503 via the drain passage 407, and is thenmixed with the oxygen-off gas flowing through the oxygen off-gas drainpassage 503. The mixture of the discharged hydrogen gas and the oxygenoff-gas is fed to the combustor 510 through the gas-liquid separator508. The combustor 510 houses a platinum catalyst 512. In the combustor610, hydrogen and oxygen contained in the mixed gas are caused to reactwith each other by combustion, so that the content of the hydrogen inthe mixed gas is further reduced. The mixed gas emitted from thecombustor 510 is then discharged or released to the atmosphere.

While the flow of the hydrogen gas during the normal operating conditionof the fuel cell system has been described above, there will bedescribed flow of the hydrogen gas during a low-temperature startupcondition of the fuel cell system.

In general, a pressure at which the hydrogen-gas absorbing alloydischarges hydrogen increases as the temperature of the hydrogen-gasabsorbing alloy increases, and decreases as the temperature of thehydrogen-gas absorbing alloy decreases. Thus, the hydrogen-gas absorbingalloy tank 200 is less likely to discharge hydrogen as the temperaturethereof decreases. It is therefore necessary to rapidly heat thehydrogen-gas absorbing alloy tank 200 by a heater or the like during thelow-temperature startup condition of the fuel cell 100, in order tofacilitate discharging of hydrogen from the hydrogen-gas absorbing alloytank. However, the use of the heater for heating the hydrogen-gasabsorbing alloy tank requires a great amount of electric energy, and isthus undesirable or inappropriate if the fuel cell system is to beinstalled on a motor vehicle.

In view of the above, the fuel cell system of this embodiment utilizesthe pump 410 to forcedly draw hydrogen out of the hydrogen-gas absorbingalloy tank 200, instead of heating the hydrogen-gas absorbing alloy tankby the heater.

FIG. 3 shows a flowchart illustrating an example of a control routineexecuted by the controller 50 when the fuel cell system of FIG. 1 isstarted.

Upon the start of the fuel cell system, the controller 50 executes stepS102 as shown in FIG. 3 to open the shut-off valve 202 of thehydrogen-gas absorbing alloy tank 200, and the shut-off valves 102 and104 of the fuel cell 100. Next, the controller 50 executes step S104 toread the pressure level of the hydrogen gas detected by the pressuresensor 400. Then, the controller 50 executes step S106 to determinewhether the detected pressure level of the hydrogen gas exceeds apredetermined reference pressure.

If the ambient temperature is sufficiently high, and the hydrogen-gasabsorbing alloy tank 200 discharges hydrogen at a sufficiently highpressure that is higher than the predetermined reference pressure, thecontroller 50 proceeds to step S114 to place the fuel cell system in thenormal operating condition as described above. In step S114, theshut-off valve 402 disposed in the main-stream passage 401 and theshut-off valve 408 disposed in the circulation passage 403 are opened,while the shut-off valve 412 disposed in the bypass passage 405 and theshut-off valve 414 disposed in the drain passage 407 are closed. Thecontroller 50 then proceeds to step S116 to drive the pump 410 at anormal speed, to thereby realize circulation of the hydrogen gas asdescribed above.

If the ambient temperature is comparatively low, on the other hand, thehydrogen-gas absorbing alloy tank 200 is less likely to dischargehydrogen and the pressure of the discharged hydrogen is lower than thepredetermined reference pressure. In this case, the controller 50proceeds to step S108 to place the fuel cell system in thelow-temperature startup operating condition. In step S108, the shut-offvalve 402 disposed in the main-stream passage 401, the shut-off valve408 disposed in the circulation passage 403, and the shut-off valve 414disposed in the drain passage 407 are closed, while the shut-off valve412 disposed in the bypass passage 405 is opened. The controller 50 thenproceeds to step S110 to drive the pump 410 at a high speed, so that asufficient amount of hydrogen gas absorbed in the hydrogen-gas absorbingalloy can be drawn out of the hydrogen-gas absorbing alloy tank 200,even in the case where the temperature of the hydrogen-gas absorbingalloy tank 200 is relatively low and the hydrogen gas is discharged at arelatively low pressure. The hydrogen gas drawn from the hydrogen-gasabsorbing alloy tank 200 is initially introduced to the main-streampassage 401, and then flows through the bypass passage 405 andcirculation passage 403 in this order. The hydrogen gas is then returnedto the main-stream passage 401, and is then supplied to the fuel cell100. The hydrogen gas supplied to the fuel cell 100 is subjected to theelectrochemical reaction in the fuel cell 100. The resulting hydrogenoff-gas is then discharged to the circulation passage 403. Since theconcentration of the impurities in the hydrogen off-gas increases withtime, the shut-off valve 414 is periodically opened to discharge thehydrogen off-gas through the drain passage 407.

The controller 50 keeps the fuel cell system in the above-describedlow-temperature startup operating condition until it is determined instep S112 that the pressure of the hydrogen gas discharged from thehydrogen-gas absorbing alloy tank 200 exceeds the predeterminedreference pressure. After a whole following the start of the fuel cellsystem, the heat exchanger (not shown) comes to operate satisfactorily,so as to heat the hydrogen-gas absorbing alloy housed in thehydrogen-gas absorbing alloy tank 200. As a result, the temperature ofthe hydrogen-gas absorbing alloy is increased, and the hydrogen-gasabsorbing alloy tank 200 becomes able to discharge hydrogen gas at asufficiently high temperature. Consequently, the pressure of thehydrogen gas exceeds the reference pressure level, and the controller 50proceeds to step S114 to shift the fuel cell system to the normaloperating condition. In step S114, the shut-off valve 402 disposed inthe main-stream passage 401 and the shut-off valve 408 disposed in thecirculatory passage 403 are opened, and the shut-off valve 412 disposedin the by-pass passage 405 and the shut-off valve 414 disposed in thedrain passage 407 are closed. Then, the controller 50 proceeds to stepS116 to drive the pump 410 at the normal speed.

In the low-temperature startup operating condition, the fuel cell systemutilizes the pump 410 for drawing hydrogen stored in the hydrogen-gasabsorbing alloy tank 200, without requiring a great amount of electricenergy.

In the fuel cell system of the present embodiment, the same pump 410 isused for circulating the hydrogen gas during the normal operatingcondition and for drawing the hydrogen gas from the hydrogen-gasabsorbing alloy tank 200 during the low-temperature startup operatingcondition. Thus, the common use of the pump 410 leads to reduced spacefor installation and reduced weight of the system.

In the present embodiment, the pump 410 is able to change its speed ofrotation so as to change its flow rate of the hydrogen gas, dependingupon whether the pump 410 operates to circulate the hydrogen gas, oroperates to draw the hydrogen gas from the hydrogen-gas absorbing alloytank 200. Namely, the pump 410 requires relatively small power forcirculating the hydrogen gas in the hydrogen-gas flow system, since thecompression ratio of the pump 410 (i.e., the ratio of a dischargepressure to an intake pressure of the pump 410) is relatively low. Onthe other hand, the pump 410 requires relatively large power for drawingthe hydrogen gas form the hydrogen-gas absorbing alloy tank 200, sincethe compression ratio of the pump 410 is relatively high.

While the flow of the hydrogen gas during the low-temperature startupoperating condition of the fuel cell system has been described above,there will be described a condition of the fuel cell system when it isstopped.

In the fuel cell 100, the impurities, such as water vapor and nitrogen,leak from the oxygen electrode side to the hydrogen electrode sidethrough the electrolyte layer, as described above. Accordingly, thehydrogen gas circulated during the normal operating condition contains acertain amount of these impurities. If the operation of the fuel cellsystem is subsequently stopped, the hydrogen-gas absorbing alloy tank200 is accordingly stopped, and the temperature within the tank 200 islowered. In this condition, a pressure within the hydrogen-gas absorbingalloy tank 200 may also decrease to a negative level, depending upon thetemperature of the tank 200. In this case, hydrogen gas flows in areverse direction from the main-stream passage 401 or the bypass passage405 to the outlet of the hydrogen-gas absorbing alloy tank 200. Ifnormal shut-off valves are used as the shut-off valve 402 disposed inthe main-stream passage 401 and the shut-off valve 412 disposed in theby-pass passage 405, such reverse flow of the hydrogen gas toward theoutlet of the hydrogen-gas absorbing alloy tank 200 cannot be completelyprevented. As a result, hydrogen gas remaining in portions of thehydrogen-gas flow system that are closer to the fuel cell 100 than theshut-off valves 402, 412 leaks into a portion of the hydrogen-gas flowsystem closer to the hydrogen-gas absorbing alloy tank 200, through theshut-off valves 402, 412, and the leaking hydrogen gas then flows intothe hydrogen-gas absorbing alloy tank 200. Since the hydrogen gasflowing into the tank 200 contains impurities, such as nitrogen andwater vapor, the impurities are also introduced into the hydrogen-gasabsorbing alloy tank 200. In this case, the impurities may possiblyaffect the hydrogen-gas absorbing alloy housed in the hydrogen-gasabsorbing alloy tank 200.

In view of the above-described problem, the shut-off valves 402, 412used in this embodiment are provided with a function of preventing orinhibiting reverse flow of the hydrogen gas. By using the shut-offvalves with the reverse-flow preventing function, the hydrogen gascontaining the impurities is prevented or inhibited from leaking intothe hydrogen-gas passage on the side of the hydrogen-gas absorbing alloytank 200 through the shut-off valves 402, 412, even when reverse flow ofthe hydrogen gas toward the tank 200 takes place upon a stop of theoperation of the fuel cell system. It is thus possible to protect thehydrogen-gas absorbing alloy in the hydrogen-gas absorbing alloy tank200.

While the condition of the fuel cell system upon a stop of the operationof the system has been described above, the operation of the fuel cellsystem when it is in an abnormal state will be hereinafter described.

If any abnormality, such as a failure in the pressure reducing valve404, arises in the fuel cell system, the pressure of the hydrogen gassupplied to the fuel cell 100 increases to an excessively high level,which may cause a problem to the fuel cell 100. In view of this problem,the present embodiment is provided with the relief valve 416 disposed inthe relief passage 409 that diverges from the main-stream passage 401.The relief valve 416 is opened to discharge a certain amount of hydrogengas to the atmosphere outside the vehicle, when the pressure of thehydrogen gas as measured in a portion of the main-stream passage 401located between the pressure reducing valve 404 and the fuel cell 100 israised to be equal to or higher than a predetermined level. Preferably,an outlet of the relief valve 416 is provided at a position that permitsthe hydrogen gas to be discharged toward a road surface, so that thehydrogen gas thus discharged will not stay at a certain location. Withthe relief valve 416 thus located, the hydrogen discharged is likely tobe diffused in the atmosphere.

Upon occurrence of a collision of the vehicle with another vehicle orobject, or a malfunction of a control system, the fuel cell system maysuffer from leakage of the hydrogen gas in the very worst case. In thefuel cell system of this embodiment, if vibrations caused by, forexample, the collision of the vehicle or the malfunction of the controlsystem are detected, the controller 50 operates to automatically closethe shut-off valve 202 disposed in the hydrogen-gas absorbing alloy tank200 and the shut-off valves 102, 104 disposed in the fuel cell 100. Inthis condition, the supply of the hydrogen gas is stopped, thuspreventing the leakage of the hydrogen gas from the fuel cell system.

Second Embodiment

FIG. 4 schematically shows the construction of an on-board fuel cellsystem according to a second embodiment of the invention. While thehydrogen-gas absorbing alloy tank 200 is used as a hydrogen-gas supplyin the fuel cell system of the first embodiment, a high-pressurehydrogen-gas tank 300 is used as a hydrogen-gas supply in the fuel cellsystem of the second embodiment.

The high-pressure hydrogen-gas tank 300 is filled with a high-pressurehydrogen gas, and a shut-off valve 302 is attached to the bottom of thetank 300. The shut-off valve 302 is opened so as to discharge hydrogengas having a pressure of about 20 MPa to about 35 MPa.

The fuel cell 100 of the fuel cell system of the second embodiment isidentical in construction with the fuel cell 100 of the firstembodiment, and therefore no explanation of the fuel cell 100 will beprovided.

As shown in FIG. 4, the fuel cell system of the present embodimentincludes a hydrogen gas passage, an oxygen gas passage and thecontroller 50. Since the oxygen gas passage is identical with the oxygenpassage of the fuel cell system according to the first embodiment, noredundant description about the oxygen gas passage will be provided.

The hydrogen-gas flow system of the fuel cell system of the secondembodiment includes a main-stream passage 401 that extends from anoutlet of the high-pressure hydrogen-gas tank 300 to the inlet of thefuel cell 100, a circulation passage 403 through which hydrogen gasreturns from the outlet of the fuel cell 100 to the main-stream passage401 via a pump 410, a drain passage 407 for discharging impuritiescontained in the circulating hydrogen gas, and a relief passage 409 fordischarging the hydrogen gas when the pressure of the hydrogen gas inthe fuel cell system is excessively high. In this embodiment in whichthe high-pressure hydrogen-gas tank 300 is used as the hydrogen-gassupply, a high-pressure hydrogen gas may be released from the tank 300irrespective of the operating temperature thereof. Thus, no bypasspassage 405 as provided in the first embodiment is provided in the fuelcell system of the second embodiment, since there is no need to drawhydrogen gas upon a low-temperature start of the system as in the caseof the hydrogen-gas absorbing alloy tank 200.

As shown in FIG. 4, a shut-off valve 302 is disposed at the outlet ofthe high-pressure hydrogen-gas tank 300, and a pressure reducing valve418, a heat exchanger 420, a pressure reducing valve 422, and agas-liquid separator 424 are disposed at selected positions in themain-stream passage 401. Also, a shut-off valve 102 is disposed at theinlet of the fuel cell 100. In addition, a shut-off valve 104 isdisposed at the outlet of the fuel cell 100, and a gas-liquid separator406, a pump 410, and a check valve 426 are disposed at selectedpositions in the circulation passage 403. Like the first embodiment, ashut-off valve 414 is disposed in the drain passage 407, and a reliefvalve 416 is disposed in the relief passage 409.

The controller 50 receives a signal (representing the pressure ofhydrogen gas in the main-stream passage 401) from a pressure sensor 400,and controls the operation of each of the valves 102, 104, 302, 414,pump 410, and a compressor 504 disposed in an oxygen-gas supply passage501. FIG. 4 does not show control lines, or the like, which indicateconnection between the controller 50 and respective components of thefuel cell system.

Initially, flow of hydrogen gas will be described in detail. Since flowof the oxidizing gas is identical with that in the first embodiment, noexplanation on the flow of the oxidizing gas will be provided.

Under control of the controller 50, the shut-off valve 302 for thehigh-pressure hydrogen-gas tank 300, and the shut-off valves 102, 104for the fuel cell 100 are basically opened during operations of the fuelcell system, and are closed when the fuel cell system is stopped.

When the fuel cell system is in a normal operating condition, theshut-off valve 414 of the drain passage 407 is closed while the shut-offvalves 302, 102, 104 are opened. As in the case of the first embodiment,the relieve valve 416 is normally held in the closed position exceptwhen the pressure of the hydrogen gas become excessively high.

During a normal operation of the fuel cell system, the controller 50keeps the shut-off valve 302 in the open position as described above, sothat the hydrogen gas is discharged from the high-pressure hydrogen-gastank 300. The discharged hydrogen gas is supplied to the fuel cell 100through the main-stream passage 401. The hydrogen gas thus fed to thefuel cell 100 is subjected to the above-indicated electrochemicalreaction in the fuel cell 100, and the resulting hydrogen off-gas isthen discharged from the fuel cell 100. The discharged hydrogen off-gasis returned to the main-stream passage 401 through the circulationpassage 403, to be supplied again to the fuel cell 100. Like the firstembodiment, the pump 410 disposed in the circulation passage 403 isdriven or actuated to forcedly feed the hydrogen off-gas into themain-stream passage 401. Namely, the hydrogen gas circulates through themain-stream passage 401 and the circulation passage 403 when the fuelcell system is in the normal operating condition. A check valve 426 forpreventing reverse flow of the circulating hydrogen gas is disposed in aportion of the circulation passage 403 located between the pump 410 anda joint of the circulation passage 403 and the main-stream passage 401.

The hydrogen gas discharged from the high-pressure hydrogen-gas tank 300has a pressure ranging from about 20 MPa to about 35 MPa, as describedabove. This pressure level is far higher than that of the hydrogen gasdischarged from the hydrogen-gas absorbing alloy tank 200 in the firstembodiment. Accordingly, if the discharged hydrogen gas is directly fedto the fuel cell 100, the fuel cell 100 will be damaged due to the highpressure of the hydrogen gas. In the second embodiment, therefore, twopressure reducing valves, namely, a first pressure reducing valve 418and a second pressure reducing valve 422, are disposed at selectedpositions in the main-stream passage 401. Namely, in the presentembodiment, the pressure of the high-pressure hydrogen gas is reduced intwo steps at the two pressure reducing valves 418, 422, while thehydrogen gas pressure is reduced only once in the first embodiment. Morespecifically, the first pressure reducing valve 418 reduces the pressureof the discharged hydrogen gas from a level ranging from about 20 MPa toabout 35 MPa to a level ranging from about 0.8 MPa to about 1 MPa. Thenthe second pressure reducing valve 422 reduces the pressure of thedischarged hydrogen gas from a level ranging from about 0.8 MPa to about1 MPa to a level ranging from about 0.2 MPa to about 0.3 MPa.

When the pressure of the high-pressure hydrogen gas is reduced by thefirst pressure reducing valve 418 from the level of about 20 MPa toabout 35 MPa to the level of about 0.8 MPa to about 1 MPa, the hydrogengas is rapidly expanded (i.e., the volume of the hydrogen gas is rapidlyincreased) about 50 times, and the temperature of the hydrogen gas israpidly reduced. If the hydrogen gas having the reduced temperature isdirectly supplied to the fuel cell 100, the temperature within the fuelcell 100 is also reduced, resulting in insufficient catalyticactivities. In this condition, the electrochemical reaction does noteffectively proceed in the fuel cell 100, resulting in deterioration ofthe power generating operation of the fuel cell 100. In view of thisproblem, a heat exchanger 420 is disposed between the first and secondpressure reducing valves 418, 422. The heat exchanger 420 functions toheat hydrogen gas whose temperature has been rapidly reduced due to itsexpansion, thereby to supply the hydrogen gas having a sufficiently hightemperature to the fuel cell 100. The heat exchanger 420 is suppliedwith cooling water heated by the fuel cell 100, which is not illustratedin FIG. 4, so that heat exchange between the warmed cooling water andthe cooled hydrogen gas takes place in the heat exchanger 420. In thismanner, the hydrogen gas whose temperature has been lowered passesthrough the heat exchanger 420, so that the hydrogen gas can be suppliedto the fuel cell 100 with its temperature being raised to a sufficientlyhigh level. Consequently, the temperature in the fuel cell 100 isincreased to a level high enough to promote the above-indicatedelectrochemical reaction, thus permitting an appropriate powergenerating operation of the fuel cell system.

As is understood from the aforementioned description, the hydrogen gasflowing through the main-stream passage 401 has a relatively lowtemperature. If the hydrogen gas having the relatively low temperatureis mixed with the hydrogen off-gas returned to the mainstream passage401 through the circulation passage 403, the moisture contained in thehydrogen off-gas is likely to be concentrated, resulting in apossibility that the hydrogen gas in the form of a gas-liquid mixture issupplied to the fuel cell 100. To avoid this possibility, the presentembodiment is provided with a gas-liquid separator 424 that is disposedin a portion of the main-stream passage 401 located between the inlet ofthe fuel cell 100 and the joint of the main-stream passage 401 and thecirculation passage 403. The gas-liquid separator 424 functions toseparate the moisture contained in the mixed hydrogen gas into a liquidcomponent and a gas component (i.e., water vapor). The gas-liquidseparator 424 removes the liquid component of the moisture, and suppliesonly the gas-component (e.g., water vapor) to the fuel cell 100 togetherwith other gas components in the hydrogen gas. With this arrangement,there is no possibility of a failure or problem in the power generatingoperation of the fuel cell 100 due to the liquid component of themoisture contained in the hydrogen gas.

While the flow of the hydrogen gas during the normal operation conditionof the fuel cell system has been described above, there will bedescribed flow of hydrogen gas during a startup operating condition ofthe fuel cell system.

When the operation of the fuel cell system is stopped, impurities, suchas nitrogen, permeate from the oxygen electrode side to the hydrogenelectrode side through the electrolyte layer within the fuel cell 100,and diffuse at the hydrogen electrode side of the fuel cell 100.Consequently, the impurities, such as nitrogen, are contained not onlyin the oxidizing-gas channels but also the hydrogen-gas channels in thefuel cell 100. Upon a start of the operation of the fuel cell system,therefore, it is necessary to enable the fuel cell 100 to perform anappropriate power generating operation within a short period of time, byremoving the impurities from the hydrogen-gas channels and filling thesechannels with hydrogen gas.

The impurities existing in the hydrogen-gas channels may be removed upona start of the fuel cell system by, for example, causing a purge gas,such as an inert gas, to flow into the hydrogen-gas channels, thereby topush the impurities out of the channels. However, this method ofremoving the impurities requires installation of an inert-gas tank onthe vehicle for supplying the purge gas, which results in undesirableincreases in the required space and weight of the fuel cell system.

In the light of the above-described problem, it may also be consideredto directly introduce hydrogen gas into the hydrogen-gas channels so asto push the impurities out of the channels. With this method, however,it takes a long time by the time when the hydrogen gas pushes theimpurities out of the hydrogen-gas channels, and the output voltage ofthe fuel cell reaches a desired level. If the hydrogen gas dischargedfrom the fuel cell is discarded or released to the atmosphere forremoval of the impurities for such a long time, the gas released to theatmosphere may contain a high concentration of hydrogen, which may causean environmental problem.

In view of the above-described situation, upon a start of the fuel cellsystem of the present embodiment, the hydrogen gas is introduced intothe hydrogen-gas channels as described above, and the pump 410 forcirculating hydrogen gas is driven or actuated to cause forced flow ofthe hydrogen gas in the hydrogen-gas flow system. Thus, the impuritiespresent in the hydrogen-gas channels in the fuel cell 100 are uniformlymixed with the hydrogen gas flowing into the channels.

FIG. 5 shows a flowchart illustrating an example of a control routineexecuted by the controller 50 when the fuel cell system of the presentembodiment is started.

Upon a start of the fuel cell system, the controller 50 executes stepS202 as shown in FIG. 5 to open the shut-off valve 302 of thehigh-pressure hydrogen-gas tank 300 and the shut-off valves 102 and 104of the fuel cell system 100, which valves 302, 102, 104 has been closed.In this condition, hydrogen gas is discharged from the high-pressurehydrogen-gas tank 300. The discharged hydrogen gas is then fed to themain-stream passage 401. Next, the controller 50 executes step S204 tooperate the pump 410 at its normal speed, to thereby cause forced flowof the hydrogen gas through the circulation passage 403. This forcedflow of the hydrogen gas serves to move the impurities existing in thehydrogen-gas channels of the fuel cell 100, and to circulate theimpurities and the hydrogen gas so as to homogeneously mix them togetherwithin a short period of time.

For instance, if the impurities existing in the hydrogen-gas channelshave an atmospheric pressure (0.1 MPa), the hydrogen gas, whose pressurehas been reduced to 2 atm (0.2 Mpa), is caused to flow through thehydrogen-gas channels. With the pressures thus controlled, the resultinggas discharged from the fuel cell 100 contains about 50% of impuritiesand about 50% of hydrogen gas. The thus discharged gas is circulatedthrough the hydrogen-gas flow system while being stirred, so that theimpurities are uniformly dispersed in the hydrogen gas.

With the impurities and the hydrogen gas thus homogenized in the manneras described above, an equal amount of hydrogen is supplied to each ofthe hydrogen electrodes in the fuel cell 100, whereby an open-endvoltage of the fuel cell 100 can be immediately raised to apredetermined level. Then, the controller 50 proceeds to step S206 todetect the rise of the open-end voltage of the fuel cell 100 based on anoutput signal received from a voltage sensor (not shown). If thecontroller 50 detects the rise of the open-end voltage, it is determinedthat the fuel cell 100 is ready to generate power, and a load (notshown) is connected to the fuel cell 100 in step S208. The controller 50then proceeds to step S210 to open the shut-off valve 414, to therebygradually discharge the circulating hydrogen gas (i.e., the homogeneousmixture of the impurities and the hydrogen gas). Since the hydrogen gasis continuously supplied from the high-pressure hydrogen-gas tank 300 tothe main-stream passage 401, the hydrogen concentration of thecirculating hydrogen gas gradually increases.

If the controller 50 determines in step S212 that a predetermined timehas passed after opening of the shut-off valve 414, the shut-off valve414 is closed in step S214, assuming that the impurities existing in thehydrogen-gas channels have been removed to some extent, and the hydrogenconcentration of the circulating hydrogen gas has increased to asufficiently high level. Then, the fuel cell system is placed in thenormal operating condition.

Upon a start of the fuel cell system of the present embodiment, thehydrogen gas is introduced into the hydrogen-gas channels of the fuelcell 100, and the pump 410 is driven so as to forcedly circulate thehydrogen gas, as described above, thus making it possible to increasethe output voltage of the fuel cell to the desired level within a shortperiod of time. Further, the fuel cell system of this embodiment doesnot require purge gas, thereby eliminating the need for a gas tank forsupplying the purge gas, resulting in reductions in the required spaceand weight of the fuel cell system. Furthermore, the fuel cell system ofthe present embodiment does not emit hydrogen gas having a highconcentration of hydrogen, thus assuring a high degree of safety.

The shut-off valve 414 disposed in the drain passage 407 and the reliefvalve 416 disposed in the relief passage 409 are identical with those inthe first embodiment, and no explanation of these valves will beprovided herein.

MODIFIED EXAMPLES

It is to be understood that the invention is not limited to the detailsof the illustrated embodiments, but may be otherwise embodied withvarious changes, modifications or improvements, without departing fromthe scope of the invention.

In the illustrated first and second embodiments, the gas-liquidseparator 406 is disposed in the circulation passage 403. Thisarrangement may be applied to a fuel cell system that employs, as ahydrogen gas supply, a reformer for reforming a raw fuel so as toproduce hydrogen gas, instead of the hydrogen-gas absorbing alloy tank200 or the high-pressure hydrogen-gas tank 300.

In fuel cell system of the second embodiment, the gas-liquid separator424 is disposed in the main-stream passage 401. This arrangement isequally applicable to the fuel cell system of the first embodiment, andto the fuel cell system that includes the reformer as the hydrogen gassupply.

In the fuel cell system of the second embodiment, the heat exchanger 420is disposed between the pressure reducing valves 418 and 422. The heatexchanger 420 may be disposed downstream of the pressure reducing valve422. Since the fuel cell system of the first embodiment uses thepressure reducing valve 404, a suitable heat exchanger may be disposeddownstream of the pressure reducing valve 404 as needed.

In the fuel cell system of the second embodiment, the operation of thefuel cell system is controlled according to the control routine as shownin FIG. 5 when the fuel cell system is started. The same control routinemay be used for controlling the fuel cell system of the firstembodiment, and the fuel cell system in which the reformer is used asthe hydrogen gas supply. In the case where the control according to theroutine of FIG. 5 is performed on the fuel cell system of the firstembodiment while the system is in a low-temperature startup condition,the pump 410 is initially driven to draw the hydrogen gas from thehydrogen-gas absorbing alloy tank 200. Subsequently, the open/closedpositions of the shut-off valves 402, 408, 412 are switched or changed,and the pump 410 is operated to circulate the impurities remaining inthe hydrogen-gas channels together with the hydrogen gas drawn from thetank 200, so that the impurities are homogeneously dispersed ordistributed in the hydrogen gas.

1. An on-board fuel cell system adapted to be installed on a motorvehicle, comprising: a hydrogen-gas storage device capable ofdischarging a hydrogen gas stored therein at a predetermined pressure; afuel cell that is supplied with the hydrogen gas discharged from thehydrogen-gas storage device, so as to generate electric power; a passagethat connects an outlet of the hydrogen-gas storage device with an inletof the fuel cell, and allows the hydrogen gas discharged from thehydrogen-gas storage device to flow therethrough to be supplied to thefuel cell; at least one pressure reducing device disposed in the passageto reduce a pressure of the hydrogen gas discharged from thehydrogen-gas storage device; and a relief valve that is able to permitand inhibit a flow of gas therethrough upon opening and closing of therelief valve, respectively; and wherein, when a pressure of the hydrogengas in the passage between the at least one pressure reducing device andthe inlet of the fuel cell is higher than a reference pressure, therelief valve is opened so that the hydrogen gas is discharged from thepassage to the outside of the vehicle.
 2. An on-board fuel cell systemadapted to be installed on a motor vehicle, comprising: a hydrogen-gasstorage device capable of discharging a hydrogen gas stored therein at apredetermined pressure; a fuel cell that is supplied with the hydrogengas discharged from the hydrogen-gas storage device, so as to generateelectric power; a first passage that connects an outlet of thehydrogen-gas storage device with an inlet of the fuel cell, and allowsthe hydrogen gas discharged from the hydrogen-gas storage device to flowtherethrough to be supplied to the fuel cell; at least one pressurereducing device disposed in the first passage to reduce a pressure ofthe hydrogen gas discharged from the hydrogen-gas storage device; asecond passage that extends from a particular point located in the firstpassage between the at least one pressure reducing device and the inletof the fuel cell, toward an outside of the vehicle; and a relief valvethat is disposed in the second passage, and is able to permit andinhibit flow of gas therethrough upon opening and closing thereof,respectively, wherein when a pressure of the hydrogen gas that exists,in the second passage, between the relief valve and the particularpoint, is higher than a reference pressure, the relief valve is openedso that the hydrogen gas is discharged from the first passage to theoutside of the vehicle through the second passage.
 3. The on-board fuelcell system according to claim 2, wherein an outlet of the secondpassage is positioned such that the hydrogen gas discharged from thesecond passage is directed toward a road surface.
 4. The on-board fuelcell system according to claim 2, wherein the hydrogen-gas storagedevice is a high pressure hydrogen gas tank.
 5. An on-board fuel cellsystem adapted to be installed on a motor vehicle, comprising: ahydrogen-gas storage device capable of discharging a hydrogen gas storedtherein at a predetermined pressure; a fuel cell that is supplied withthe hydrogen gas discharged from the hydrogen-gas storage device, so asto generate electric power; a first passage that connects an outlet ofthe hydrogen-gas storage device with an inlet of the fuel cell, andallows the hydrogen gas discharged from the hydrogen-gas storage deviceto flow therethrough to be supplied to the fuel cell; at least onepressure reducing device disposed in the first passage to reduce apressure of the hydrogen gas discharged from the hydrogen-gas storagedevice; a relief valve that is able to permit and inhibit a flow of gastherethrough from the first passage to the outside of the vehicle, uponthe respective opening and closing of the relief valve; and a passageportion which is closer to a particular point in the first passage thanis the location of the relief valve, the particular point being locatedbetween the at least one pressure reducing device and the inlet of thefuel cell, wherein when a pressure of the hydrogen gas in the passageportion is higher than a reference pressure, the relief valve is openedso that the hydrogen gas is discharged from the particular point in thefirst passage to the outside of the vehicle.